Switching power supply is to maintain a stable output voltage by controlling a time ratio of turn-on and turn-off of a switch tube using a modern power electronics technology. Nowadays, the switching power supply is widely applied to almost all of the electronic equipments due to its advantage of small, lightweight and high efficiency, and becomes an indispensable power supply for the rapid development of the current electronic information industry.
FIG. 1 schematically illustrates a structural block diagram of a typical switching power supply in a conventional technology. As shown in FIG. 1, the switching power supply 100 may include at least an output port (not shown), a rectifier circuit 101, a filter circuit 102, a transformer T1, a capacitor C1, a load R1, a primary controller 103, and a first voltage divider circuit comprised of an up-sampling resistor R_U and a down sampling resistance R_D. The output port is configured to generate an output voltage Vout, and may be coupled with a capacitor C1 and a load R1 which are configured to filter out ripple of the output voltage Vout. An Alternating Current (AC) input signal Vin is transmitted to a primary winding n1 of the transformer T1 via a rectification of the rectification circuit 101 and a filtering of the filter circuit 102, and the secondary winding n2 of the transformer T1 is coupled with the output port. An auxiliary winding n3 of the transformer T1 outputs a feedback voltage V_AUX, and the first voltage divider circuit is configured to divide the feedback voltage V_AUX to obtain a first divided voltage FB. The primary controller 103 has a feedback port (not shown) and a power supply port (not shown), where the feedback port is configured to be input with the first divided voltage FB, the power supply port is configured to be input with a power supply voltage VDD, and the power supply voltage VDD supplies power to the primary controller 103.
A negative feedback is formed, due to the first divided voltage FB input in the feedback port reflecting a change of the output voltage Vout in real time. The primary controller 103 may control a current of the primary winding n1 of the transformer T1 according to the first divided voltage FB to indirectly control a current of the secondary winding n2 of the transformer T1 that is coupled, thereby controlling a magnitude of the output voltage Vout. Specifically, it can be achieved by controlling a duty cycle of a Pulse Width Modulation (PWM) signal generated by the primary controller 103. The transformer T1 starts to store energy when the current of the primary winding n1 of the transformer T1 increases, and the current coupled to the secondary winding n2 will continue to be transmitted to the output port when the current of the primary winding n1 is turned off. When the current of the primary winding n1 is turned off, a dissipation process of electrical energy coupled to the secondary winding n2 takes places. Such dissipation process is commonly referred to as a demagnetization process of the secondary winding n2, and a duration of the demagnetization process is referred to as a demagnetization time. Normally, the primary controller 103 may monitor the demagnetization time of the secondary winding n2 by monitoring a waveform of the first divided voltage FB.
The up-sampling resistor R_U may be disconnected for various reasons. Since one end of the up-sampling resistor R_U is normally grounded through the down sampling resistance R_D, when the up-sampling resistor R_U is disconnected, the first divided voltage FB approaches a zero potential, the negative feedback path is disconnected, the demagnetization time and the output voltage Vout of the secondary winding n2 cannot be accurately detected, and the switching power supply 100 is in an open loop state. In this situation, potential of the output voltage Vout and potential of the power supply voltage VDD will still be continuously increased. At this time, appropriate measures should be taken to protect the switching power supply 100.
There are two methods for detecting disconnection of the aforementioned up-sampling resistor R_U in the conventional technology. One detection method is to detect the power supply voltage VDD, i.e., if the VDD exceeds the overvoltage protection threshold of the power supply port, then it is determined that the up-sampling resistor R_U is open. This detection method has a disadvantage that the detection may be not in time, i.e., when the power supply voltage VDD exceeds the overvoltage protection threshold, an actual value of the output voltage Vout may have already exceeded the maximum allowable voltage of the capacitor C1 or the load R1, which may make the capacitor C1 or the load R1 burned down. The other detection method is to detect the demagnetization time of the secondary winding n2 through the first divided voltage FB, i.e., if the demagnetization time is less than a certain time threshold, then it is determined that the up-sampling resistor R_U is open. This detection method has a disadvantage that the first divided voltage FB may not be constantly zero potential due to a voltage coupling phenomenon, which makes the primary controller 103 still capable of detecting a demagnetization time greater than the certain time threshold, thereby resulting in a misjudgment.
Therefore, there is a disadvantage in the conventional technology that the detection of the disconnection of the up-sampling resistor of the switching power supply is not timely or is prone to causing a misjudgment.